fix(adapters): ground native schemas, merge-not-replace, add goose/cagent/zed

- common/mcp.go: MCPStyle enum + BuildMCPMapStyled for claude/crush/opencode/zed dialects
- common/jsonmerge.go: MergeJSONKeys (JSON-first, JSONC fallback) preserves user keys;
  fix: try plain json.Unmarshal before StripJSONComments to avoid mangling URLs with //
- common/yamlmerge.go: MergeYAMLKeys for YAML configs (goose)
- common/blocks.go: add ProvidersAsMap for opencode map-shaped provider key
- claudecode: merge settings.json (preserves hooks/permissions/env); MCPClaudeStyle (no type field)
- crush: fix mcp key (was mcpServers); MCPCrushStyle entries include type field; merge crush.json; fix import key mcp
- kilo: drop providers key (kilo has none); MCPOpencodeStyle (local/remote); merge kilo.json
- opencode: fix provider (singular map) + model (not default_model); MCPOpencodeStyle; merge jsonc; fix import key mcp
- goose: new adapter — YAML merge, extensions map, splitProviderModel, import reconstructs MCP from extensions
- cagent: new adapter — renders starter default.yaml, import returns empty
- zed: new adapter — merge settings.json, context_servers (MCPZedStyle) + agent.default_model
- docs: append Grounded harness schemas section to design spec

internal/adapter/common/jsonmerge.go

@@ -0,0 +1,34 @@
+package common
+
+import "encoding/json"
+
+// MergeJSONKeys reads existing JSON/JSONC body, overlays the given top-level
+// keys (replacing values for listed keys, preserving every other key), and
+// returns the merged JSON. If existing is empty or invalid JSON the function
+// returns just the overlay encoded fresh (2-space indent + trailing \n).
+// An overlay value of nil deletes the key from the base.
+func MergeJSONKeys(existing []byte, overlay map[string]any) ([]byte, error) {
+	base := map[string]any{}
+	if len(existing) > 0 {
+		// Try plain JSON first (avoids mangling URL strings that contain //).
+		// Only fall back to comment-stripping for JSONC content.
+		if err := json.Unmarshal(existing, &base); err != nil {
+			clean := StripJSONComments(string(existing))
+			if err2 := json.Unmarshal([]byte(clean), &base); err2 != nil {
+				base = map[string]any{}
+			}
+		}
+	}
+	for k, v := range overlay {
+		if v == nil {
+			delete(base, k)
+			continue
+		}
+		base[k] = v
+	}
+	out, err := json.MarshalIndent(base, "", "  ")
+	if err != nil {
+		return nil, err
+	}
+	return append(out, '\n'), nil
+}